1. Field of the Invention
This invention relates to a process for preparing a liquid silicone rubber base composition which is useful as a starting material for silicone rubber which has utility in the fields of electric and electronic industries, automobiles, business machines, and medical appliances.
2. Description of the Prior Art
In known liquid silicone rubber base compositions, it is the usual practice to add fillers, such as dry or wet process silica having a large specific surface area, in large amounts in order to develop high mechanical strength of cured silicone rubber. However, if fillers are added in large amounts, the fluidity of the resultant liquid silicone rubber base composition is worsened. This makes it impossible to obtain liquid silicone rubber bases having good moldability.
For increasing the fluidity of the liquid silicone rubber base, several processes have been used including a process wherein wetting agents are added so as to improve the wettability of the base, a process wherein the rubber base is diluted with base oils, and a process of adding inorganic fillers under high temperature conditions.
In the process wherein wetting agents are added, there are used, as a wetting agent, hexamethyldisilazane, cyclic dimethylsiloxane, polydimethylsiloxanes having a degree of polymerization of not higher than 30 and terminated with a hydroxyl group at ends thereof, and alkoxy group-containing silanes or alkoxy group-containing siloxanes.
As the process wherein base oils are used for dilution, there is mentioned a process wherein reinforcing silica is formulated in organopolysiloxane in large amounts and thermally treated, followed by dilution with base oils.
For the process wherein inorganic fillers are added under high temperature conditions, there is known a process wherein an inorganic filler is added to and kneaded along with a rubber base at a temperature of from 200.degree. to 300.degree. C. by use of a biaxial extruder, and then thermally treated. This process is set out, for example, in U.S. Pat. No. 4,649,005 corresponding to Japanese Patent Publication (examined) No. 4-28008.
The liquid silicone rubber base compositions obtained according to the prior art processes have, respectively, fluidity sufficient for practical applications and enable one to produce cured products which have a tear strength of not lower than 20 kgf/cm. However, these processes have the problem that once the product has been cracked, the cracks are readily spread in a tensile stress condition, resulting in the breakage of the product within a short time.